Introducing sorption coefficient through extended UNIQAC and Flory-Huggins models for improved flux prediction in forward osmosis

© 2018 Elsevier Ltd A 2D computational fluid dynamic-based finite element model was developed to describe water flux and concentration polarization in forward osmosis (FO) under steady-state conditions. The extended UNIQAC equation in the feed and the Flory Huggins equation on the membrane side were solved simultaneously to calculate the activity coefficients of the diffusion components. The ratio of the feed component activity coefficient to that on the membrane surface was defined as the sorption coefficient. Employing the sorption coefficient led to a significantly improved accuracy of the developed model in estimating water flux. The membrane structural parameter determining internal concentration polarization (ICP) should be maintained at reasonable values. Otherwise, it may significantly increase the reverse salt flux and external concentration polarization (ECP) on the active layer. This suggests the tradeoffs between the membrane structural parameter and membrane selectivity, between ECP and ICP. Water fluxes were similar in counter-current and co-current flow modes. ICP had the most significant resistance to flow, reducing the water flux up to 75%. ECP on the feed side had a limited effect on FO water flux, particularly at high flow velocity. ECP on the draw side and reverse salt flux had negligible effects on FO water flux in our modeling. Our study provides important insights into developing high-performance FO membranes with reasonably low structural parameters and improving FO water flux by reducing the key limiting factor (e.g. ICP).